1. Field of the Invention
This invention relates to a process for preparing a noble metal-containing catalyst. More particularly, this invention relates to a process for preparing zeolite catalysts containing a highly dispersed noble metal in the form of small crystallites.
2. Discussion of Prior Art
Shape-selective catalysis utilizing molecular sieves was first demonstrated by P. B. Weisz and V. J. Frilette in J. Phys. Chem., 64, page 302 (1960). Since then, the shape-selective catalytic properties of various zeolites have been extensively demonstrated. For example, N. Y. Chen and W. E. Garwood, in "Some Catalytic Properties of ZSM-5, a New Shape Selective Zeolite", Journal of Catalysis, 52, pages 453-458 (1978), described the shape-selectivity of ZSM-5. On the other hand, the use of zeolites as shape-selective supports for catalytic functions has received much less attention.
P. B. Weisz, V. J. Frilette, R. W. Maatman and F. B. Mower, in "Catalysis by Crystalline Aluminosilicates II. Molecular-Shape Reactions", Journal of Catalysis, 1, pages 307-312 (1962), described a shape-selective olefin hydrogenation catalyst comprising platinum incorporated in zeolite A. In U.S. Pat. No. 3,140,322 to V. J. Frilette and P. B. Weisz, a process is disclosed for hydrogenation using a platinum-containing zeolite. In U.S. Pat. No. 3,226,339 of V. J. Frilette and R. W. Maatman, a process is described for the preparation of a platinum- or palladium-containing zeolite catalyst. U.S. Pat. No. 3,575,045 to J. N. Miale discloses the use of a platinum-entrained zeolite A for selective hydrogenation.
A catalyst and process for selectively hydrogenating ethylene in the presence of propylene utilizing a zeolite in conjunction with a hydrogenation metal is disclosed in U.S. Pat. No. 3,496,246. N. Y. Chen and P. B. Weisz, in "Molecular Engineering of Shape-Selective Catalysts", Kinetics and Catalysis, Chem. Eng. Prog. Symp., Ser. No. 73, Vol. 63, 1967, page 86, describes a platinum catalyzed hydrogenation employing a phosphine-poisoned, platinum-exchanged sodium mordenite zeolite.
An excellent summary of the art of metal loaded zeolite catalysts and shape-selective catalysis is given in Zeolite Chemistry and Catalysts, J. A. Rabo, Ed., ACS Monograph 171 (1976). Of particular interest is Chapter 10, "Catalytic Properties of Metal-Containing Zeolites" by K. M. Minachev and Y. I. Isakov, and Chapter 12, "Shape-Selective Catalysis" by S. M. Csicsery.
Catalysts, such as ZSM-5, combined with a Group VIII metal are described in U.S. Pat. No. 3,856,872 to Morrison. It is disclosed in this patent that the catalysts be preferably incorporated in a porous matrix, such as alumina. A Group VIII (hydrogenation) metal may then be added after incorporation with the zeolite in a matrix by such means as base-exchange or impregnation. In one embodiment, the metal is added in the form of chloroplatinic acid.
U.S. Pat. No. 4,188,282 discloses particularly preferred forms of noble metal-containing zeolites, such as ZSM-5, formed by the crystallization of the zeolite from a forming solution containing noble metal ions, such as those of platinum. U.S. Pat. No. 3,462,377 to Plank et al discloses the preparation of metal-containing zeolite catalysts in which the activity of the catalyst is enhanced by steaming.
British Pat. No. 1,189,850 discloses the preparation of a noble metal containing zeolite catalyst in which a metal loaded ammonium zeolite, which has been manufactured by contacting zeolite material with ammonia and/or ammonium ions and which has been composited with one or more hydrogenation metals, is subjected to controlled oxidative calcination.
The introduction of noble metals by ion-exchange methods, such as those described in U.S. Pat. No. 3,856,872 and British Pat. No. 1,189,850, can result in serious losses of the noble metal being exchanged because of the excess amounts of noble metal-containing solution required. The co-crystallization method of U.S. Pat. No. 4,188,282 not only results in significant losses of noble metals, but requires extensive modifications to the zeolite production process.
One method for reducing the volume of noble metal solution is to add the noble metal directly to the zeolite in the mulling step, i.e., by physically intimately mixing the noble metal with the zeolite, during the catalyst synthesis process.
U.S. Pat. No. 4,312,790 to Butter et al discloses a method of preparing a noble metal-containing catalyst by incorporating a noble metal in a cationic form with a zeolite after crystallization of said zeolite, but prior to the final catalyst particle formation. The zeolite is calcined only after extrusion, i.e., after addition of the noble metal. Such catalysts have been found to be an improvement over those catalysts wherein the metal is incorporated during zeolite crystallization, or after extrusion. The catalyst thus produced also exhibits little hydrogenation-dehydrogenation activity.
There are methods known in the prior art for the redispersion of metals on deactivated catalysts. For example, U.S. Pat. Nos. 3,134,732, 3,986,982 and 4,444,895 teach the reactivation or regeneration of a metal-containing catalyst by treating the catalyst with a halide and/or halogen gas during the treatment process. However, heretofore, there has not been any disclosure or suggestion of an efficient and economical process for preparing a highly-dispersed metal-containing hydroisomerization zeolite catalyst, such as that disclosed in the present invention.